Projection systems, used for projecting an image on to a screen, use several different components for providing efficient illumination of the image display unit. Projection systems typically use a lamp to generate the illumination light, with several optical elements being disposed between the lamp and the image display unit to efficiently transfer the light from the lamp to the image display unit. The image display unit may use different mechanisms for imposing an image on the incident beam of light. For example, the image display unit may use absorption, as with a photographic slide, polarization, as with a liquid crystal display, or by the deflection of light, as with a micromechanical array of individually addressable, tiltable mirrors. Some image display units require that differently colored components of the image be imposed by splitting the light beam into beams of primary colors, imposing separate images on the primary color beams and then recombining the primary color images to produce the final image.
Image brightness is a key parameter for characterizing a projection system. Image brightness may be affected by several factors, such as the brightness of the lamp, the efficiency of collecting the light from the lamp, the efficiency of homogenizing the light beam, the efficiency of relaying the light to the image display unit, and the efficiency of imposing the image on the light beam. It is often desirable to be able to form the image to be as bright as possible. When the étendue of the projection system, however, limits the amount of light in the image, the solution to increased brightness is to use a more powerful lamp. Étendue is the product of the imager area and the solid angle determined by the f-number of the optical system. More powerful lamps are expensive and often have a shorter lifetime. Also, more powerful lamps tend to have a larger arc, which reduces the geometrical efficiency of light collection.